The present invention relates to automatic electronic or "computer" flash systems. In particular, the present invention relates to electronic flash systems having an improved performance correct exposure annunciator.
Automatic electronic flash systems include a light producing means, generally a flash tube, which is actuated to illuminate a scene being photographed. A light sensing or exposure control circuit detects the scene illumination and actuates a light terminating or light quenching means when sufficient light has been produced to properly expose a light sensitive film of an associated camera.
There is a need for an annunciator which indicates to the photographer whether sufficient light has been produced to properly expose the film. Correct exposure annunciators which perform this function in response to a single indicative of correct exposure are described in U.S. Pat. No. 3,706,911 by Dennis J. Wilwerding, and Ser. No. 603,565 by James R. Adams, Jr. and Dennis J. Wildwerding, and Ser. No. 603,564 by James R. Adams, Jr. and in the above mentioned application by John D. Dick and Dennis J. Wilwerding, all of which are assigned to the same assignee as this application.
In the past, correct exposure annunciators have operated in response to a signal which is indicative of premature termination of the light flash by the light terminating means. These correct exposure annunciators, therefore, have not indicated correct exposure if the illumination received by the light sensing or exposure control circuit is only slighly less than the quantity required to cause premature flash termination.
Automatic electronic flash systems typically have an automatic control level which is set to provide "full rated illumination" out to the distance at which "full light flash" occurs. A "full light flash" is a light flash which is not prematurely terminated by the terminating means. Instead, the light flash is terminated because the voltage on the main flash capacitor eventually drops to a level which will no longer support conduction through the flash tube. The terminating means has not operated because the exposure control circuit has not received sufficient light before most of the energy has been dissipated from the main flash capacitor.
Beyond the distance at which a full light flash occurs, the flash illumination drops off as the square of the distance. Acceptable photographs, however, can be obtained with illumination down to one f/stop below the "full rated illumination".
The disadvantage of the previous correct exposure annunciator circuits is that the annunciator only indicates correct exposure when "full rated illumination" is provided. The annunciator does not indicate, however, that an acceptable picture is still possible if the illumination is only slightly less than "full rated illumination". The photographer may not be aware of the fact that the correct exposure annunciator does not indicate all conditions in which acceptable photographs can be obtained.